In several cases, users prefer to interact with a computer using an electronic pen or stylus instead of mouse or keyboard. For example, when a user needs to compose a handwritten document on a paper or a whiteboard. One difficulty associated with a handwritten document is that of archiving it into digital form providing a way to edit this archived version. Prior art approaches typically involve scanning the document.
Some methods and systems rely on a pattern to analyze an encoded surface and to determine localize a pen. Those existing patterns have some disadvantages. Among these are:                1. Existing patterns have an excessively high number of dots per unit area (i.e. too high a dot density).        2. Dots used in known dot patterns tend to be too small to be correctly printed using a typical home printer.        3. Known dot patterns tend to distract a reader, thus making reading a document more difficult.        4. The document cannot be printed at same time as the dot pattern.        5. Know dot patterns distribute dots evenly on the paper.        
For a pattern with a high dot density, the image analysis is difficult enough so that the device position cannot be returned in real time using a low cost and low power processor. Additionally, where the dots are small (i.e. about 50 microns in diameter) the dot pattern is very noise susceptible. Thus, any dirt on the paper or substrate makes it difficult for a pen to read the dot pattern.
The inability to print the dot pattern and the document concurrently is of particular significance. In the prior art, one first prints a pattern on the paper. Then, one prints the document over the already-printed pattern. A difficult that can arise is a mismatch in alignment between the pattern and the document. To overcome this difficulty, the document is often printed with a special ink to avoid covering the pattern.
Another disadvantage of the prior art is that the user is obligated to press the pen against the paper to allow the pattern is recognized. This means that the pen includes mechanical parts and sensors in the pen to detect when the pen's tip contacts the paper, causing several problems in relation to the pattern recognition.